Precision weapons play a significant role in battlefield success by providing improved weapon accuracy and allow a much lighter launch vehicle for corresponding effectiveness with an unguided system. One example of a precision weapon is a missile.
Laser guided weapons are used in a variety of applications which often require accurate target closing rate and closing distance information to successfully direct the weapon to its target. Poor or corrupted closing rate and/or range information may cause warhead misalignment, premature detonation or targeting error.
In active guidance systems a transmitter onboard the laser guided weapon facilitates ranging—determination of the distance between the weapon and the target. By measuring the round trip signal travel time for a signal transmitted via the transmitter and reflected back from the target a range estimate is obtained. The active guidance systems are primarily advantageous in areas where no man-assisted guidance is possible. However, these types of systems are usually less accurate than systems that include targeting by an operator.
Semi-active systems utilize a remote platform with an illumination source or transmitter. Operation of the illumination source or transmitter is usually man-assisted in order to achieve the best accuracy in the targeting. However, the illumination source may also be automatically operated from the remote platform. In a semi-active laser guidance system, the illumination source radiates a beam of pulsed energy toward a target or a chosen spot on the target. The beam is typically generated and transmitted from a laser designator platform. The illumination source marks the target for the weapon which homes in on the reflected laser energy to strike the target. Semi-active guidance systems enable the laser guided weapon to sense the direction of the target and to direct the course of the weapon in the direction of the target. However, these types of systems usually lack the ability to accurately determine range when a target is in motion. Range estimated by the state-of the art semi-active guidance systems is often grossly inaccurate and can result in inefficient weapon guidance, increased fuel consumption and mistimed weapon detonation. To facilitate the determination of the remaining distance before hitting the target, many laser guided weapons also includes a position indicating system or other additional guidance means, i.e., inertial reference units.
U.S. Pat. No. 6,204,801 to Sharka et al. discloses a system for determining the range between a missile and a target adapted for use with a semi-active missile system. The receiver system in the missile includes two receivers in order to be able to produce range information for the target on the basis of a frequency modulated periodic signal from the illuminating system. The semi-active missile system is a radar based system and the disclosed solution would not be applicable in a laser based system. The solution requires that both the missile and the target are illuminated by the same illumination system, which is incompatible with the concept of semi-active laser targeting.